Wind turbine diffuser

ABSTRACT

The invention concerns a diffuser for a whine turbine, in particular a large-dimension wind turbine mounted on a mast ( 2 ) and comprising a wind-driven propeller ( 3 ) equipped with blades ( 4 ) as well as an alternator converting wind kinetic energy into electric, power. The invention is characterized in that it consists of a circular element ( 7 ) enclosing the ends ( 9 ) of the blades ( 5 ), and consisting of a skin ( 14 ) made of a stretched textile membrane associated with a rigid internal and/or external framework ( 15 ) supporting the loads and enabling to stretch the membrane and forming it.

The invention relates to a diffuser for a wind turbine, in particular awind turbine of large dimensions mounted on a mast and having awind-driven propeller as well as an alternator co-operating with thepropeller to supply electrical energy.

Specialists have been trying to recover wind energy for a long time, itsadvantages being that it is clean, i.e. does not create thermal orchemical pollution, and inexhaustible.

However, these advantages are compensated by a series of disadvantages,linked in particular to the dispersed and intermittent nature of wind;it also well known that wind turbine “parks” occupy a large amount ofspace and cannot operate without creating a noise nuisance.

Furthermore, the wind turbines currently in use are often fitted withpropellers having radial blades and a horizontal shaft similar to thoseused to propel aircraft but generally much bigger as a rule. Suchpropellers conventionally co-operate with industrial dynamos oralternators with geared drives, which makes them heavy, expensive andlow in output.

As a result of these disadvantages, the market for wind turbines has nottaken off as might have been expected in recent years and the potentialfor development in this field today is very high.

In order to overcome these disadvantages, a more robust wind turbine hasbeen proposed more recently, disclosed in document FR-2 793 528, whichis more compact but produces the same output and is less noisy thanpreviously proposed wind turbines.

A wind turbine of this type is mounted on a vertical mast and has awind-driven propeller fitted with helical blades which are inclinedtowards the upstream end, supported on a hub with a horizontal axis, aswell as an alternator co-operating with the propeller to supplyelectrical energy, and which is also equipped with a magnetic rotorsecured to the hub and a stator with a magnetic coil adjacent to therotor and secured to a fixed underframe.

This wind turbine is also equipped with a static diffuser, whichconsists, on the one hand, of a fixed circular element of relativelyshort length mounted concentrically with the hub on arms integral withthe underframe and co-operating, with a slight clearance, with the endsof the blades, at a slight clearance therefrom, and on the other hand, arounded leading edge followed by a thick body and a thin divergenttrailing edge.

It has been proven from numerous tests conducted in wind tunnels thatthis wind turbine has aerodynamic characteristics which are very muchsuperior to earlier conventional wind turbines.

This result is due to the optimised profile of the rotor blades and thepresence of the divergent diffuser.

A diffuser of this type conventionally has a skin, which is what impartsits contour, and an internal structure which is designed to absorb loadand accommodate the ends of support arms enabling the diffuser to becentred around the wind turbine blades.

In the case of small wind turbines (up to a few meters in diameter), itis possible to use solutions based on boiler technology or conventionalmanufacturing technologies, using either thin composite skins inconjunction with an internal box with a combined foam/epoxy glassstructure or thick rigid skins, in particular of plastics, steel orDuralumin® which also fulfil a structural function.

However, these conventional technologies are out of the question when itcomes to wind turbines of bigger diameters (which may be up to severaltens of metres): in practice, steel skins with a thickness of 1 to 2 mmwould create much too heavy a mass and would incur too high a cost,whilst still requiring the presence of an appropriate rigid internalstructure.

For economic reasons, a diffuser made from a composite material wouldnot be feasible either, because the cost of machining and raw materialswould be far too high.

Consequently, the known solutions cannot be transposed tolarge-dimension wind turbines of the type described above because theywould not be viable on an industrial scale.

To date, no technologies have ever been proposed that would reduce theweight and cost of such diffusers.

The objective of the present invention is to fill that gap.

To this end, it relates to a diffuser for a wind turbine, in particulara wind turbine of large dimensions mounted on a mast and having awind-driven propeller fitted with blades, as well as an alternator, forconverting kinetic wind energy into electricity.

For the purposes of the invention, a diffuser of this type ischaracterised in that it comprises a circular element enclosing the endsof the blades and has a skin in the form of a stretched textile membranein conjunction with a rigid internal and/or external framework tosupport loads and enable the membrane to be kept taut and to impart therequisite shape to it.

It should be pointed out that the invention may advantageously be usedfor a wind turbine of the type described above but could also be adaptedfor use with any wind turbine system with a turbine casing.

The textile membrane proposed by the invention preferably has a densityin the order of 1 kg/dm³.

The use of such a textile membrane has proved to offer variousadvantages: in effect, there are currently membranes of this type on themarket which have high mechanical properties, high resistance to UVradiation and good resistance to bad weather conditions; in addition,manufacturers offer maintenance facilities in the event of tearing.

It should also be pointed out that the tension of this textile membrane,produced by means of the internal framework, can be controlled over timein order to compensate for any creep which might occur in the materials.

By virtue of another characteristic of the invention, the textilemembrane has a lagging, thereby enabling it to be reinforced, especiallyin the region disposed in proximity to the blades.

Large depressions can be generated locally across the entire contour ofthe diffuser, i.e. its internal and external surfaces, depending on themain direction of the wind relative to the rotation axis of the blades,which can cause the textile membrane to become detached from theframework.

It is vital that this phenomenon does not occur in the region located inproximity to the blades in order to avoid any risk of tearing.

For this reason, it is crucial to provide either a reinforcing laggingor means for securing the textile membrane to the internal framework atthis level.

As a result of another feature of the invention, the rigid frameworkconsists of a series of identical radiating ribs, which areintrinsically closed with a planar curve forming a wing-shaped contourmatching the contour of the diffuser and constituting its internal andexternal surfaces.

These radiating ribs may advantageously be made from a metal such assteel or aluminium, or alternatively from timber or a compositematerial.

By virtue of another characteristic of the invention, the rigidframework also has a series of peripheral members linking the radiatingribs and, if necessary, a series of stiffening elements enabling thethickness of the diffuser contour to be kept constant.

These members and these stiffening elements may be made from the samematerials as those used for the radiating ribs. In a first variant ofthe invention, the radiating ribs as well as the peripheral members aremade from steel tubes.

In this variant, adjacent peripheral members may advantageously belinked by stiffening elements provided in the form of steel sectionsdisposed in the plane of the radiating ribs along the internal andexternal faces of the diffuser and/or transversely thereto.

Connecting steel rods, also intended to fulfil a stiffening function,may optionally be incorporated parallel with the peripheral members inorder to link the radiating ribs.

In a second variant of the invention, the radiating ribs are provided inthe form of aluminium sections, the cross-section of which isspecifically of a Ω shape, whilst the peripheral members are provided inthe form of aluminium tubes.

In this variant, the various aluminium tubes forming the peripheralmembers may advantageously be inter-linked by other aluminium tubesserving as stiffening elements.

It should be pointed out that the framework used in this second variantis essentially similar to that used for the first variant of theinvention but has the advantage of being lighter in weight due to thenature of the materials used.

In a third variant of the invention, the radiating ribs are provided inthe form of timber sections, in particular T-shaped, and the peripheralmembers are provided in the form of carbon beams, specifically with arectangular cross section, linked in pairs by cores of plywood inparticular, on either side of the radiating ribs so as to definebox-type spars.

These box-type spars preferably have cut-outs at their median part inorder to reduce the weight of the rigid framework as far as possibleand, as a general rule, are linked in pairs by stainless steel cables.

In a fourth variant of the invention, the radiating ribs are provided inthe form of elements with a rectangular cross-section, in particular,made from a laminated-bonded composite material, and the peripheralmembers are provided in the form of beams, in particular with arectangular cross-section made from a laminated-bonded compositematerial, linked in pairs on either side of the radiating ribs by aseries of rings rigidly secured to one another and also made from alaminated-bonded composite material.

In this variant of the invention, the internal and external faces ofeach of the radiating ribs are preferably linked by a series of rings,rigidly secured to one another and made from a laminated-bondedcomposite material, serving as stiffening elements.

It is also of advantage to provide connecting timber strips linking theradiating ribs, incorporated parallel with the peripheral members.

The characteristics of the wind turbine diffuser proposed by theinvention are described in more detail below with reference to examplesillustrated in the appended drawings, although these should not beconstrued as restrictive in any way:

FIG. 1 shows a perspective view of a wind turbine fitted with a staticdiffuser as proposed by the invention,

FIG. 2 is a view showing a partial axial section of this wind turbine,

FIG. 3 is a partial perspective view illustrating the mounting of thestatic diffuser,

FIG. 4 is an axionometric view depicting a detail of the framework of adiffuser used for the first variant of the invention,

FIG. 5 is an axionometric view similar to that shown in FIG. 4 butdepicting the framework of a diffuser used for the second variant of theinvention,

FIG. 6 is an axionometric view similar to that shown in FIG. 4 butdepicting the framework of a diffuser used for the third variant of theinvention,

FIG. 7 is an axionometric view similar to that shown in FIG. 4 butdepicting the framework of a diffuser used for the fourth variant of theinvention.

The wind turbine 1 illustrated in FIGS. 1 and 2 is mounted on a verticalmast 2 and has a wind-driven propeller 3 fitted with helical blades 5inclined towards the upstream side.

The blades 5 are supported by a hub 4 with a horizontal axis.

The wind turbine 1 also has an alternator, not illustrated in thedrawings, which co-operates with the propeller 3 to generate electricalenergy.

This alternator is fitted with a magnetic rotor, fixed to the hub 4, anda stator with magnetic coils adjacent to the rotor and secured to afixed underframe 6.

The rotor and the stator are not illustrated in the drawings.

The wind turbine also has a static diffuser 7 provided in the form of afixed circular element of relatively short length.

This diffuser 7 is mounted concentrically with the hub 4 on arms 8integral with the underframe 6 and co-operates with the ends 9 of theblades 5 at a slight clearance therefrom.

As illustrated in FIG. 2, the static diffuser 7 has an aerodynamicprofile with a rounded leading edge 10 followed by a thick body 11 and athin divergent trailing edge 12.

As illustrated in FIG. 1, the divergent trailing edge 12 is supported bystays 13 distributed in radial planes.

As may be seen from FIG. 2, this configuration enables a divergence tobe created in the air flow downstream of the diffuser as indicated byarrows F and F′.

The design of the static diffuser proposed by the invention will now bedescribed with reference to FIGS. 3 to 6.

It should be pointed out that in these drawings, the blades 6 of thepropeller 3 are not illustrated and are merely schematically indicatedby their axis XX′.

In FIG. 3, the diffuser 7 consists of a skin 14 made from a textilematerial with a very low density. This skin 14 is stretched around arigid framework 15, which imparts its shape and enables it to be centredaround the blades 6.

The framework 15 is essentially made up, on the one hand, of a series ofradiating ribs 16, all of which are identical, and on the other hand, aseries of peripheral members 17, the purpose of which is to link theradiating ribs 16 and receive the ends of the arms 8 on which thediffuser 7 is mounted.

In FIG. 3, the radiating ribs 16 are provided in the form ofintrinsically closed elements with a planar curve forming a wing-shapedcontour corresponding to the contour of the diffuser 7 as illustrated inFIG. 2.

These radiating ribs 16 therefore constitute the internal surface 18 andexternal surface 19 of the diffuser 7.

As illustrated in FIG. 4, the radiating ribs 16 ₁ as well as theperipheral members 17 ₁ are provided in the form of steel tubes.

The tubes forming the peripheral members 17 ₁ are linked to one anotherby various stiffening elements in order to maintain a constant thicknessof the contour of the diffuser 7.

In FIG. 4, these elements are steel sections 20 disposed in the planesof the radiating ribs 16 ₁ and stainless steel cables 21 disposedtransversely to these planes.

These various stiffening elements 20, 21 also co-operate with connectingsteel rods 22, linking the radiating ribs 16 ₁, disposed parallel withthe peripheral members 17 ₁.

In FIG. 5, the radiating ribs 16 ₂ are aluminium sections having across-section with a Ω shape, whereas the peripheral members 17 ₂ arealuminium tubes.

The various adjacent tubes 17 ₂ on the internal surface 18 or on theexternal surface 19 of the diffuser 7 or on either side of the radiatingribs 16 ₂ are linked by aluminium tubes 23 disposed in the plane of theradiating ribs 16 ₂ or transversely thereto in a pyramid design toimpart rigidity to the internal framework 15 of the diffuser 7.

In FIG. 6, the radiating ribs 16 ₃ are timber T-shaped sections whilstthe peripheral members 17 ₃ are carbon beams with a rectangularcross-section.

With the exception of the front and downstream ends of the radiatingribs 16 ₃, the beams 17 ₃ are linked in pairs on either side of theseribs 16 ₃ by plywood cores 24 with cut-outs 25 in their median part.

The beams 17 ₃ and the plywood cores 24 thus define box-type sparsenabling the thickness of the contour of the diffuser 7 to bemaintained.

In FIG. 6, adjacent box-type spars are linked to one another byrigidity-imparting cables 26 disposed obliquely relative to the plane ofthe radiating ribs 16 ₃.

In FIG. 7, the radiating ribs 16 ₄ are elements with a rectangularcross-section with a timber base made from a laminated-bonded compositematerial, whereas the peripheral members 17 ₄ are beams with arectangular cross-section made from a similar material.

The beams 17 ₄ are linked in pairs on either side of the radiating ribs16 ₄ by a series of rings 27 rigidly secured to one another and alsomade from a laminated-bonded composite material.

In FIG. 7, the internal face 18 and external face 19 of the radiatingribs 16 ₄ are also linked to one another by a series of rings 28 rigidlysecured to one another and also made from a laminated-bonded compositematerial.

These rings 28 also co-operate with the timber strips 29 linking theradiating ribs 16 ₄, disposed parallel with the peripheral members 17 ₄.

It should be pointed out that the fixing arms 8 in this configurationare not secured directly to the peripheral members 17 ₄ at the level oftheir outer ends 8 ₁ but are mounted at this level on mounting plates 30which are in turn secured to the rings 27, 28.

1. Diffuser for a wind turbine, in particular a wind turbine of largedimensions mounted on a mast (2) and having a wind-driven propeller (3)fitted with blades (5) and an alternator for converting kinetic windenergy into electricity, characterised in that it comprises a circularelement (7) surrounding the ends (9) of the blades (5) and comprising askin (14) in the form of a stretched textile membrane in conjunctionwith a rigid internal and/or external framework (15) supporting load andenabling the membrane to be kept taut and impart the requisite shape toit.
 2. Diffuser as claimed in claim 1, characterised in that the textilemembrane (14) has a density in the order of 1 kg/dm³.
 3. Diffuser asclaimed in claim 1, characterised in that the textile membrane (14) isprovided with a lagging enabling it to be reinforced, in particular inits zone located in proximity to the blades (5).
 4. Diffuser as claimedin claim 1, characterised in that the rigid framework (15) comprises aseries of identical radiating ribs (16), intrinsically closed and havinga planar curve with a wing-shaped contour corresponding to the contourof the diffuser (7) and constituting the internal surface (18) andexternal surface (19) thereof.
 5. Diffuser as claimed in claim 4,characterised in that the rigid framework (15) comprises: a series ofperipheral members (17) enabling the radiating ribs (16) to be linkedand, if necessary, a series of stiffening elements enabling thethickness of the contour of the diffuser (7) to be kept constant. 6.Diffuser as claimed in claim 5, characterised in that the radiating ribs(16 ₁) and the peripheral members (17 ₁) are steel tubes.
 7. Diffuser asclaimed in claim 5, characterised in that the radiating ribs (16 ₂) areprovided in the form of aluminium sections with a cross-section with a ashape, whilst the peripheral members (17 ₂) are aluminium tubes. 8.Diffuser as claimed in claim 5, characterised in that the radiating ribs(16 ₃) are provided in the form of timber sections which are T-shaped inparticular, and the peripheral members (17 ₃) are carbon beams, inparticular with a rectangular cross-section, linked in pairs by cores(24) in particular of plywood, on either side of the radiating ribs (16₃) so as to define box-type spars.
 9. Diffuser as claimed in claim 8,characterised in that the box-type spars have cut-outs (25) at theirmedian part.
 10. Diffuser as claimed in claim 5, characterised in thatthe radiating ribs (16 ₄) are provided in the form of elements, inparticular with a rectangular cross-section, made from alaminated-bonded composite material, and the peripheral members (17 ₄)are beams, in particular with a rectangular cross-section made from alaminated-bonded composite material linked in pairs on either side ofthe radiating ribs (16 ₄) by a series of rings (27) rigidly secured toone another and also made from a laminated-bonded composite material.11. Diffuser as claimed in claim 10, characterised in that the internalface (18) and external face (19) of each of the radiating ribs (16 ₄) islinked by a series of rings (28) rigidly secured to one another and madefrom a laminated-bonded composite material.